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Old 10-23-2007, 05:40 PM
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Carb tuning issues - 750 speed demon

I know I've read other posts about this, but every engine is different...so here we go.
When I was looking for a new carb I did a lot of homework. For my setup, a fairly well built up 351w with a 5 speed, the 750cfm speed demon, mech. secondaries, and annular boosters seemed to be the best choice. I am running a dual plane intake which should be helping my situation out. At idle I am getting a lot of smoke. Very rich condition. The intake smells of fuel after taking the carb off and after running the truck for a bit and cutting it off it still smokes. The problem seems to happen when it's at operating temps. I've been through the setup procedure over again to ensure I'm not missing anything, but my problem persists even after trouble-shooting. Just looking for some advise from some of the more experienced ones out there.
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Old 10-23-2007, 07:12 PM
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I also have a Holley 750 cfm double pumper, on a chevy 355. I had the same problem, my problem was the idle mixture, it was too rich. Make sure your engine is at operating temp. before setting it, a colder engine requires more fuel. If you have a big cam, the throttle plates will be open too far, rendering the idle mixture screws useless, i had to drill my throttle butterflies in order to get them closed enough. Besides the rich idle, do you have any other problems??
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Old 10-23-2007, 10:52 PM
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The demon has an idle-eze adjustment to make up for the big cam instead of the drilling the butterflies method. The problem is that I see an improvement in idle quality with this adjustment, but I still get smoke. I'm going to try some things out tomorrow so we will see. Other than the idle troubles I feel like the 750 demon gives me more giddy-up over the holley 670 avenger style carb. I definitely enjoy the mech. secondaries and I feel more top end power.
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Old 10-24-2007, 09:39 PM
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I am concerned about the air bleeds. I was told by a local shop to look into those being too open. My question is about the speed demons setup in that mine was shipped with no restriction on these air bleeds whatsoever. Does this make sense? Would that cause a rich condition? One thing to note is that at idle I hear the carb sucking in air through these and I was told this was a bad thing. I'm sure I'll get this problem behind me, but I am tired of getting conflicting information from the local boys..hah.
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Old 10-25-2007, 04:02 PM
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I've got less smoke but problems with it not wanting to idle down. Any ideas?
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Old 10-25-2007, 04:34 PM
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One of the best tools to use when setting any carb,especially a Demon is a vacuum gauge.I had all sorts of troubles getting mine set on the small block.When I jumped up to running a big block,I went back to the Ol Faithful Holley.

Concerning the sticking problem.Are you having this problem with the air filter on or off?I had the lower base on a Z28 of mine catching the linkage and keeping it revving higher.
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Old 10-25-2007, 04:47 PM
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Air cleaner off.
I have a vacuum gauge I am using, but I'm not fully sure what I'm looking for.
Thanks.
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Old 10-26-2007, 04:05 PM
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I've got it to stop sticking open, but I'm getting my smoke again. Pretty tricky. Hah, any suggestions?
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Old 10-26-2007, 05:40 PM
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Bah, it started back with the not idling down again. I really want to get this to work.
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Old 10-26-2007, 05:59 PM
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I'm not familiar with the Demon carb, I know a similiar problem on a Holley can be caused by using the wrong power valve. Does the Demon have a power valve similiar to a Holley?
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Old 10-26-2007, 06:48 PM
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They have what they call blow proff power valves
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Old 10-26-2007, 07:08 PM
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If you have low vacuum at idle, usually because of cam specs, the power valve will be open at idle and make the engine very rich. Check your vacuum at idle then see what number power valve you have. On a Holley it's written on the power valve. I think I've been told the power valve should be 1/2 of your idle vacuum -2. For example if your idle vacuum is 16 the power valve should be a number 6. I may be a little off on that number, but I'm sure someone will correct me if I'm wrong......lol.
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Old 10-26-2007, 07:34 PM
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idle

Power valves have nothing to do with the idle circut, they are 2 different circuts in the carb..


What is your initial timing set at????

Keith
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Old 10-26-2007, 07:58 PM
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Go back to school Keith
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Old 10-26-2007, 10:25 PM
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If you are right and the power valve should be 1/2 - 2...I have the wrong power valve. Right now the engine pulls 13hg and it has the 6.5 in it. Initial timing is set at 16*, but I've been trying the range from 10*-20*. Cam is about .550 lift and 220 duration at .050.
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